Lithiated silicon/carbon composite materials and method for producing the same

ABSTRACT

The invention relates to composite materials comprising lithiated or partially-lithiated graphite or graphene, and silicon having particles sizes from about 1 μm to about 100 μm, and that have an electrochemical rest potential less than about 2 V measured against Li/Li + , wherein graphitic material is mixed with silicon powder in a molar ratio of 9:1 to 1:9 and with lithium powder to an amount of the lithium in the composite material in the range of about 10 molar % to 100 molar % of the stochiometrically maximally possible lithium absorption, and to methods for production thereof.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.15/546,746 dated Jul. 27, 2017, which is the National Stage ofInternational Patent Application Number PCT/EP2016/051680, filed on Jan.27, 2016, which in turn claims benefit of German Application Number 102015 201 461.4, filed on Jan. 28, 2015, the disclosures of which areincoroporated herein by reference.

TECHNICAL FIELD

The invention relates to composite materials comprising Li, Si and C,wherein the electrochemical rest potential of these compounds is belowapproximately 2 V, preferably below 1 V, measured against Li/Li⁺.

BACKGROUND

Silicon is one of the most-promising anode materials for lithiumbatteries of the next generation. The semimetal has an extremely highabsorbing capacity for lithium via the mechanism of alloy formation: forexample, the alloy Li₂₂Si₅ has a maximum theoretical capacity of 4200Ah/kg, which is more than ten times higher than that of the graphitescurrently built into lithium ion batteries (372 Ah/kg). Unfortunately,the high volume changes (>300%) during charging/discharging result in apulverizing and a separation of the connection from the current arresterso that a poor reversibility and an extremely rapid capacity drop takeplace. For graphene, a reversible capacity of up to 740 mAh/gcorresponding to a composition LiC₃ is considered to be possible,according to the document US 2015/0000118 A1.

The electrochemical properties of silicon can be improved by reducingthe particle sizes in the submicron range, by alloy formation with otherelements, nanostructuring of electrodes or by admixing components whichbuffer the volume change (e.g., carbon).

When using silicon as anode material in lithium batteries there isanother problem in that very high, irreversible losses are recordedduring the first charge/discharge cycle. They can be traced backprimarily to the content of foreign elements such as, e.g., oxygen,hydrogen and inorganic carbon (e.g., carbonate). The foreign elementsreact irreversibly with lithium to electrochemically inactive productssuch as lithium oxides, lithium carbonate, lithium carbide, lithiumhydroxide, etc.

An improved anode material mainly based on silicon is sought.

This material should be able to be produced by a commerciallyadvantageous process.

DESCRIPTION OF THE INVENTION

The problem is solved with composite materials of this inventioncomprising lithiated or partially-lithiated graphite or graphene andsilicon having particle sizes from about 1 μm to about 100 μm, andhaving an electrochemical rest potential less than about 2 V measuredagainst Li/Li⁺. Such composite materials can also comprise metalliclithium. Further, such composite materials can have a coating layercomprising functional groups or molecular constituents that have reactedwith lithium that was available on the composite materials' surfaces,including wherein the coating layer is applied with one or more gaseousor liquid coating means.

Also provided are methods for producing composite materials comprisingcombining (i) a graphitic material with particle sizes from about 5 μmto about 200 μm and a silicon powder with particle sizes from about 1 μmto about 100 μm in a molar ratio of 9:1 to 1:9 with (ii) a lithiumpowder with a particle sizes from about 5 μm to about 500 μm under inertgas conditions to form a combination and mechanochemically convertingthe combination in a temperature range of about 0° C. to about 120° C.under inert gas or in a vacuum, into the composite materials, whereinthe amount of lithium in the composite materials is in the range ofabout 10 molar % to about 100 molar % of the stochiometrically maximallypossible lithium absorption. Included are such methods wherein themechanochemically converting is conducted in a temperature range ofabout 20° C. to about 100° C., and/or such methods comprising alsocombining metallic lithium particles having sizes between about 5 μm toabout 500 μm to form the combination. Also provided are such methodscomprising subjecting the produced composite materials to a temperatureof about 150° C. to about 350° C. for about 5 minutes to about 24 hours.Also provided are such methods additionally comprising applying to theproduced composite materials a coating layer comprising functionalgroups or molecular constituents such that the functional groups ormolecular constituents react with lithium available on the compositematerials' surfaces, including applying the coating layer via a gaseouscoating agent or a liquid coating agent, and wherein the gaseous coatingagent is selected from N₂, CO₂, CO, O₂, N₂O, NO, NO₂, HF, F₂, PF₃, PF₅,and POF₃, and the liquid coating agent is selected from carbonic acidesters, vinyl ethylene carbonate, ethylene carbonate, propylenecarbonate, dimethyl carbonate, diethyl carbonate, ethyl methylcarbonate, fluoroethylene carbonate; lithium chelateoborate solutions inorganic solvents, and inorganic compounds.

Also covered by the present invention is the use of composite materialsthat comprise lithiated or partially-lithiated graphite or graphene andsilicon particles having sizes from about 1 μm to about 100 μm, and thathave an electrochemical rest potential less than about 2 V measuredagainst Li/Li⁺, as anode material in galvanic cells with non-aqueouselectrolytes and/or as anode materials in lithium batteries.

The composite materials that comprise lithiated or partially-lithiatedgraphite or graphene, and silicon particles having sizes between 1 to100 μm, and that have an electrochemical rest potential less than about2 V measured against Li/Li+, can be used as anode material in galvaniccells with non-aqueous electrolytes.

Preferably, the composite materials that comprise lithiated orpartially-lithiated graphite or graphene, and silicon particles havingsizes between 1 to 100 μm, and that have an electrochemical restpotential less than about 2 V measured against Li/Li+, can be used asanode material in lithium batteries.

The amount of the lithium in the composite materials of this inventionis in the range of about 10 to 100 molar % of the stochiometricallymaximally possible lithium absorption (LiC₆ and Li₂₂Si₅ are thethermodynamically Li-richest stable phases at room temperature). Theproduction of the composite materials of the invention takes place forexample via a grinding process which can be optionally combined with atempering process. To this end, the graphitic material (for examplegraphite powder with particle sizes between 5 and 200 μm or graphenepowder) is mixed with silicon powder (particle size 1 to 100 μm) in amolar ratio of 9:1 to 1:9 and with lithium powder (particle size 5 to500 μm) under inert gas conditions (e.g., Ar) and is subsequentlycompressed or ground. At this time Li-graphite intercalates with thecomposition LiC_(o) (o=e.g., 6 or 12), meaning that lithium atoms areintercalated in graphite in a molar ratio of 1:6 or 1:12, surprisinglyform at first, whereas no or only an entirely subordinate reaction oralloy formation takes place between silicon and lithium metal.

The mechanically induced conversion takes place in the temperature rangebetween 0 and 120° C., preferably 20 to 100° C. either in vacuum orunder an atmosphere whose components do not react or only acceptablyslowly react with metallic lithium, silicon and/or lithium graphiteintercalation compounds. This is preferably either dry air or a noblegas, especially preferably argon.

The lithium is added in powdery form comprising particles with anaverage particle size between about 5 and 500 μm preferably 10 and 200μm. Coated powders such as, e.g., a stabilized metallic powder offeredby the FMC company (Lectromax powder 100, SLMP) with a lithium contentof at least 97 wt % or, for example, a powder coated with alloy-formingelements and with metallic contents of at least 95 wt %(WO2013/104787A1) are used. Non-coated lithium powders with a metalliccontent of ≥99 wt % are especially preferably used. For a use in thebattery area the purity regarding metallic contaminations must be veryhigh. Among other things, the sodium content must not be >200 ppm. TheNa content is preferably ≤100 ppm, especially preferably ≤80 ppm.

All powdery graphite qualities, both those from naturally occurring ones(so-called “natural graphite”) as well as synthetically/industriallyproduced types (“synthetic graphites”) can be used as graphite.Macrocrystalline flake graphites as well as amorphous ormicrocrystalline graphites can be used. As regards graphenes, there isbasically no limitation. However, the oxygen content should be below 5wt %, preferably below 1 wt %. The silicon powder has a content of atleast 80 wt % Si, preferably at least 90 wt % Si; and the remaindersubstantially comprises oxygen.

The conversion (that is the lithiation or partial lithiation) of thegraphite or graphene takes place during admixing, compression and/orgrinding of the two components lithium powder and graphite-or graphenepowder in the presence of the Si powder. The grinding can take place bymortar and pestle on a laboratory scale. However, the conversionpreferably takes place in a mechanical mill, for example, a rod mill,oscillating mill or ball mill. The conversion is especiallyadvantageously carried out in a planet ball mill. To this end, e.g., theplanet ball mill Pulverisette 7 premium line from the Fritsch companycan be used on a laboratory scale. When using planet ball mills veryadvantageously short reaction times of <10 h, frequently even <1 h canbe surprisingly achieved.

The mixture of lithium powder and graphite powder is preferably groundin the dry state. However, a fluid which is inert to both substances canalso be added up to a weight ratio of up to 1:1 (sum of Li+C+Si:fluid).The inert fluid is preferably a non-aqueous hydrocarbon solvent, e.g., aliquid alkane or alkane mixture or an aromatic hydrocarbon mixture. Thevigorousness of the grinding process is dampened and the graphiteparticles are groundless strongly by the addition of solvents.

The grinding time is a function of various requirements and processparameters:

-   weight ratio of grinding balls to product mixture-   type of grinding balls (e.g., hardness and density)-   intensity of the grinding (frequency of rotation of the grinding    plate)-   reactivity of the lithium powder (e.g., type of coating)-   weight ratio of Li:C-   product-specific material properties-   desired particle size, etc.

The conditions can be discovered by a person skilled in the art bysimple optimizing experiments. In general, the grinding times fluctuatebetween 5 minutes and 24 hours, preferably between 10 minutes and 10hours. After the end of the mechanochemical conversion a composite ispresent consisting of lithiated or partially lithiated graphite/graphenepowder, largely unchanged Si-Powder and lithium metallic remainders.

These lithiated or partially lithiated composite powders are “active” toenvironmental conditions (air and water) and to many functionalizedsolvents (e.g., NMP) and liquid electrolyte solutions, i.e. they reactor decompose upon rather long exposure times. When stored in normal airthe contained lithium reacts under the development of hydrogen tothermodynamically stable salts such as lithium hydroxide, lithium oxideand/or lithium carbonate. In order to at least largely avoid thisdisadvantage, the lithiated or partially lithiated composite powders canbe stabilized by a second process step, a coating method. To this endthe lithiated or partially lithiated composite powder is passivated witha gaseous or liquid coating agent. The coating agents used contain,compared to metallic lithium and lithium graphite intercalationcompounds or lithium graphene intercalation compounds, reactivefunctional groups or molecular constituents and they therefore reactwith the lithium available on the surface. The conversion of thelithium-containing surface zone takes place under the formation oflithium salts such as, e.g., lithium carbonate, lithium chloride,lithium hydroxide, lithium alcoholates, lithium carboxylates, etc.)which are non-reactive or only slightly reactive to air (thereforethermodynamically stable). In this coating procedure the greatest partof the lithium which is not present on the particle surface (e.g. of theintercalated component) remains in active form, i.e., with anelectrochemical potential of ≤1 V vs. Li/Li⁺. Such coating means areknown from lithium ion battery technology as in-situ film producers(also designated as a SEI producers) for the negative electrodes and aredescribed, for example, in the following survey article: A.Lex-Balducci, W. Henderson, S. Passerini, Electrolytes for Lithium IonBatteries, in Lithium-Ion Batteries, Advanced Materials andTechnologies, X. Yuan, H. Liu and J. Zhang (Hrsg.), CRC Press BocaRaton, 2012, p. 147-196. Coating agents used are cited by way of examplein the following. Suitable gases are N₂, CO₂, CO, O₂, N₂O, NO, NO₂, HF,F₂, PF₃, PF₅, POF₃and the like. Liquid coating agents used are, forexample: carbonic acid esters (e.g., vinylene carbonate (VC), vinylethylene carbonate (VEC) ,ethylene carbonate (EC), propylene carbonate(PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methylcarbonate (EMC), fluoroethylene carbonate (FEC)); lithium chelateoboratesolutions (e.g., lithium bis(oxalato)borate (LiBOB), lithiumbis(salicylato)borate (LiBSB), lithium bis(malonato)borate (LiBMB),lithium difluorooxalatoborate (LiDFOB) as solutions in organic solvents,preferably selected from: oxygen-containing heterocycles such as THF,2-methyl-FHF, dioxolan; carbonic acid esters (carbonates) such asethylene carbonate, propylene carbonate, dimethyl carbonate, diethylcarbonate and/or ethyl methyl carbonate; nitriles such as acetonitrile,glutarodinitrile; carboxylic acid esters such as ethylacetate,butylformiate and ketones such as acetone, butanone; sulfur-organiccompounds (e.g., sulfites, (vinylethylene sulfite, ethylene sulfite);sulfones, sultones and the like); N-containing organic compounds (e.g.,pyrrole, pyridine, vinylpyridine, picolines, 1-vinyl-2-pyrrolidinone);phosphoric acid; organic, phosphorus-containing compounds (e.g., vinylphosphonic acid); fluorine-containing organic and inorganic compounds(e.g., partially fluorinated hydrocarbons, BF₃, LiPF₆, LiBF₄),silicon-containing compounds (e.g., silicon oils, alkyl siloxanes) amongothers.

When using liquid coating agents the coating process generally takesplace under an atmosphere of inert gas (e.g., argon protectiveatmosphere) at temperatures between 0 and 150° C. In order to increasethe contact between the coating agent and the lithiated or partiallylithiated composite powder, mixing or agitating conditions areadvantageous. The necessary contact time between the coating agent andthe lithiated or partially lithiated composite powder is a function ofthe reactivity of the coating agent, of the prevailing temperature andof other process parameters. In general, times between 1 minute and 24hours are appropriate.

The coating not only improves the handling properties and the safety inthe production of electrodes (generally anodes) but also the propertiesof use in an electrochemical battery cell. When pre-coated anodematerials are used, the in situ formation of an SEI (solid electrolyteinterface) is eliminated upon contact of the lithiated or partiallylithiated composite anode material with the liquid electrolyte of thebattery cell. The anode filming brought about outside of theelectrochemical cell by pre-coating corresponds in its properties to aso-called artificial SEI. In the ideal case the otherwise necessaryforming process of the electrochemical cell is eliminated or it is atleast simplified.

The composite products lithiated or partially lithiated and stabilizedaccording to the above-described method can be used to produce batteryelectrodes. For this, they are mixed and homogenized under inert or dryspatial conditions with at least one binder material and optionally witha conductivity-improving additive (e.g., blacks or metallic powder,e.g., Ni powder or Ni foam) and with an organic solvent and thisdispersion is applied by a coating method (casting method, spin coatingor airbrush method) onto a current arrester and dried. The stabilizedlithiated or partially lithiated composite powders produced according tothe method of the invention are surprisingly not very reactive toN-methylpyrrolidone (NMP) and to other functionalized, organic solvents.Therefore, they can be processed with NMP and the binder materialpolyvinylidene fluoride (PVdF) to a castable or sprayable dispersion.Other examples for suitable binder materials are, among others:carboxymethyl cellulose (CMC), polyisobutylene (e.g., OPPANOL of theBASF Company), alginic acid.

In a preferred variant of the method according to the invention thedescribed, non-coated, lithiated or partially lithiated composites aresubjected following the mechanochemical conversion to a temperature stepat temperatures between 100 and 350° C., preferably between 150 and 250°C. During a tempering time of 5 minutes to 24 h a conversion takes placebetween the lithiated or partially lithiated graphite-/graphenecompounds and optionally any elementary (metallic) lithium still presentto alloys of lithium and silicon (lithium silicides, e.g. Li₇Si₃). Uponmaintaining sufficient storage times at a certain temperature it ispossible to extract all lithium intercalated in the graphite or grapheneand to use it for the production of the lithium silicides. In thismanner a silicide composite is produced in the extreme case whichconsists of lithium-free or very lithium-poor graphite/graphene andlithium silicides. The exact composition results from the stoichiometryof the reaction batch.

In a variant of a method the silicide composite materials of theinvention can also be produced by mixing separately produced, powderyLiC_(o) intercalation compounds (o=30 to 6) with silicon powder (1-100μm particle size) in the desired molar ratio and in possibly asubsequent thermolysis phase (130 to 350° C. for 5 min to 24 h,preferably 140-300° C. Use of the term “LiC_(o) intercalation compounds(o=30 to 6)” means that lithium atoms are intercalated in graphite in amolar ration from 1:30 up to 1:6.

It was surprisingly found that the non-coated, lithiated or partiallylithiated composites as well as the silicide composites in contact withelectrolytic solutions and carbonate solvents are more stable than purelithium-graphite intercalation compounds. As an example, the followingbeginnings of an exothermal decomposition reaction can be observed forcomposites produced in the molar ratio Si:Li:C=1:2.7:3.9, when stored ina mixture of ethylene carbonate/ethyl methyl carbonate (EC/EMC, 1:1 w/w)in DSC experiments with the Radex system of the Systag company:

-   Non-coated lithium graphite intercalation compound LiC₆ (Li    content=8.8 weight %) T_(onset)=130° C.-   Non-thermolyzed Li/Si/C—composite with the composition    Li_(2.7)SiC_(3.9) (Li content=28.5 weight %): T_(onset)=170° C.-   At 150° C. 4 hours thermolyzed Li/Si/C—composite with the    composition Li_(2.7)SiC_(3.9) (Li content=28.5 weight %):    T_(onset)=140° C.-   At 250° C. 10 hours thermolyzed Li/Si/C—composite with the    composition Li_(2.7)SiC_(3.9) (Li content=28.5 weight %):    T_(onset)=150° C.

In spite of the lithium concentration in the composite materialsaccording to the invention, which is significantly higher in comparisonto LiC₆, the materials according to the invention have an improvedthermal stability compared to an EC/EMC mixture.

Furthermore, it was surprisingly found that the composite materials arenot self-igniting in air as a rule. This is the opposite of the behaviorof non-coated LiC₆.

The electrochemical rest potential of the composite material of theinvention is below about 2 V, preferably below 1 V measured againstLi/Li⁺.

The composite materials according to the invention can be used ashigh-capacitive anode materials for galvanic cells with non-aqueouselectrolytes, for example lithium batteries.

EXAMPLE 1

A mixture consisting of:

-   1.80 g (64 mmol) Si powder (supplier Wacker, Si content 89.4 wt %,    D₅₀=58 μm)-   1.16 g (167 mmol) Li powder (Rockwood Lithium, non-coated, Li    content >99 wt %,    -   D₅₀=105 μm)-   3.00 g (250 mmol) graphite powder (SLP 30 from the Timcal company)    was ground together with 26 ZrO₂ balls, diameter 3 mm 4 h at 400 rpm    in a reversion operation in a planet ball mill Pulverisette P 7 with    a zirconium oxide grinding cup from the Fritsch company in a glove    box filled with Ar. This composition of mixture is within the    claimed ranges:-   64 mmol Si can accommodate max. 282 mmol Li according to the    limiting stoichiometry Li₂₂Si5-   250 mmol graphite can accommodate max. 42 mmol Li according to the    limiting stoichiometry LiC₆.    The maximum Li uptake capacity of this quantity of mixture is    therefore 324 mmol, which is 52 molar % of the lithium metal    quantity (167 mmol) used.

5.56 g of a golden-brown powder was obtained. The phases LiC₁₂, LiC₆, Simetal, and Li metal can be identified in this product by powder x-raydiffractometry. Graphite and Li/Si alloys cannot be identified.

The product surprisingly proved to be non-self-igniting in air. Itvigorously reacts with N-methylpyrrolidone after a short time.

EXAMPLE 2

1.05 g of a mixture produced according to example 1 are thermolyzed inclosed steel autoclaves in Ar protective gas for 4 hours at 150° C.Subsequently, the following phases can be identified by XRD: lithiumsilicides, graphite and Si (reduced intensity). Metallic lithium cannotbe identified.

The product is not self-igniting in air. It reacts mildly with NMP atroom temperature.

EXAMPLE 3

1.16 g of a mixture produced according to example 1 are thermolyzed inclosed steel autoclaves in Ar protective gas for 10 hours at 250° C.Subsequently, the following phases can be identified by XRD: lithiumsilicides (elevated intensity), graphite and Si (greatly reducedintensity). Metallic lithium cannot be identified.

The product is not self-igniting in air. It reacts only extremely weaklywith NMP at room temperature.

The examples show the production of Li/C/Si composites with a highlithium content (52 wt % of maximum lithium capacity) and theirqualitative composition. A thermal post-treatment improves the stabilityto reactive solvents shown, for example, on a mixture with N-methylpyrrolidone.

We claim:
 1. Composite materials comprising lithiated orpartially-lithiated graphite or graphene and silicon having particlesizes from about 1 μm to about 100 μm, and having an electrochemicalrest potential less than about 2 V measured against Li/Li⁺.
 2. Thecomposite materials of claim 1 that additionally comprise metalliclithium.
 3. The composite materials of claim 1 having a coating layercomprising functional groups or molecular constituents that have reactedwith lithium that was available on the composite materials' surfaces. 4.The composite materials of claim 3 wherein the coating layer is appliedwith one or more gaseous or liquid coating means.
 5. A method forproducing composite materials comprising combining (i) a graphiticmaterial with particle sizes from about 5 μm to about 200 μm and asilicon powder with particle sizes from about 1 μm to about 100 μm in amolar ratio of 9:1 to 1:9 with (ii) a lithium powder with a particlesizes from about 5 μm to about 500 μm under inert gas conditions to forma combination and mechanochemically converting the combination in atemperature range of about 0° C. to about 120° C. under inert gas or ina vacuum, into the composite materials, wherein the amount of lithium inthe composite materials is in the range of about 10 molar % to about 100molar % of the stochiometrically maximally possible lithium absorption.6. The method of claim 5, wherein the mechanochemically converting isconducted in a temperature range of about 20° C. to about 100° C.
 7. Themethod of claim 5 comprising also combining metallic lithium particleshaving sizes between about 5 μm to about 500 μm to form the combination.8. The method of claim 7 further comprising subjecting the producedcomposite materials to a temperature of about 150° C. to about 350° C.for about 5 minutes to about 24 hours.
 9. The method of claim 5additionally comprising applying to the produced composite materials acoating layer comprising functional groups or molecular constituentssuch that the functional groups or molecular constituents react withlithium available on the composite materials' surfaces.
 10. The methodof claim 9 comprising applying the coating layer via a gaseous coatingagent or a liquid coating agent.
 11. The method of claim 10, wherein thegaseous coating agent is selected from N₂, CO₂, CO, O₂, N₂O, NO, NO₂,HF, F₂, PF₃, PF₅, and POF₃, and the liquid coating agent is selectedfrom carbonic acid esters, vinyl ethylene carbonate, ethylene carbonate,propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methylcarbonate, fluoroethylene carbonate; lithium chelateoborate solutions inorganic solvents, and inorganic compounds.
 12. Use of compositematerials that comprise lithiated or partially-lithiated graphite orgraphene and silicon particles having sizes from about 1 μm to about 100μm, and that have an electrochemical rest potential less than about 2 Vmeasured against Li/Li⁺, as anode material in galvanic cells withnon-aqueous electrolytes.
 13. Use of composite materials that compriselithiated or partially-lithiated graphite or graphene and siliconparticles having sizes from about 1 μm to about 100 μm, and that have anelectrochemical rest potential less than about 2 V measured againstLi/Li⁺, as anode material in lithium batteries.